The invention relates to anodic bonding.
One way a glass material may be bonded to an oxidizable material (e.g., a metal, such as silicon) or another glass material is through a process called anodic bonding. During anodic bonding, heat is applied to the materials to be bonded, and oxygen ions in the heated glass material are drawn across a junction (where the two materials contact each other) to form a chemically bonded oxide bridge between the two materials. To draw the oxygen ions across the junction, an electric field typically is applied to the materials to create a flow of charge through the materials. The materials are heated until the alkali and alkaline earth ions become mobile allowing non-bridging oxygen ions to also diffuse. In this manner, negatively charged oxygen ions flow in one direction across the junction, and positively charged ions (e.g., alkali ions, such as sodium and lithium) flow in the opposite direction across the junction.
Referring to FIG. 1, as an example, anodic bonding might be used to bond a glass substrate 10 to a metal, such as silicon 12. To accomplish this, an electrode 14 is placed on the glass substrate 10 and biased (via a DC source 20) at a negative potential relative to the potential of another electrode 16 that is placed on the silicon 12. If the film of silicon is electrically conductive, electrical contact may be made directly to the film. In this manner, the two electrodes 14 and 16 establish an electric field across the glass substrate 10 and the silicon 12.
This electric field causes the positive ions (e.g., sodium ions) of the substrate 10 to move toward the negative electrode 14 and oxygen ions of the substrate 10 to move toward the positive potential (e.g., either toward the positive electrode 16 or the film of silicon, if conductive). As a result, the oxygen ions diffuse across a junction 18 (where the two materials contact each other) into the silicon 12 and react as follows:
2Oxe2x88x92+Si= greater than SiO2+2exe2x88x92
Thus, the oxygen ions react with the silicon to form silica (SiO2), a stable oxide, which bonds the glass substrate 10 and the silicon 12 together. The amount of silica that is formed depends on the amount of charge that is supplied by the source 20.
Therefore, the rate at which the silica is formed depends on how fast charge is supplied by the source 20, or stated differently, the rate at which the silica is formed is a function of the magnitude of a current (called IBOND) that is provided by the source 20. Although the rate at which the anodic bond is formed depends on the magnitude of the IBOND current, the quality of the bond is also quite often a function of the IBOND current.
When the IBOND current has a large magnitude, the relatively slow flow rate of the glass substrate 10 causes the silica to be formed in a small area. Better bond quality is typically achieved when the IBOND current has a smaller magnitude which allows the silica to form over a much larger area.
Although a minimum amount of silica must be formed to ensure a good bond, too much silica formation may present difficulties. For example, the silicon 12 might be a thin layer that is formed on top of a substrate. As a result, forming too much silica may delaminate, or remove, the silicon layer from the substrate.
Although anodic bonding has traditionally been used to bond small materials (e.g., materials having no dimension greater than six inches) together, anodic bonding may be used to bond materials to a larger substrate. For example, anodic bonding might be used to attach glass spacer rods to oxidizable material of a face plate of a field emission display (FED). Because of the relatively large size (e.g., dimensions greater than 12 inches) of the face plate, temperature gradients cause the magnitudes of the IBOND currents to vary, depending on where the anodic bonding occurs on the face plate. As a result, even if the same potential is used to bond all sites on the face plate, the silica is formed at different rates among the different bond sites.
The invention is generally directed to anodically bonding two materials together by monitoring and controlling the amount of charge used to bond the materials.
The advantages of the invention may include one or more of the following. The amount of oxide used to bond the materials is precisely controlled, and this amount is not affected by temperature. Several pieces of one material can be bonded to another relatively large material at one time. The cost of manufacturing flat panel displays is reduced. The time required to manufacture flat panel displays is reduced. Better quality control is maintained over the anodic bonding.
Generally, in one aspect, the invention features a controller for use with an anodic bonding system that has a charge flowpath for supplying charge to bond materials together. The controller includes a switch and a circuit. The switch is configured to control a flow of the charge through the charge flowpath. The circuit is configured to monitor a rate of the flow, use the rate to determine an amount of the charge supplied for bonding, and based on the amount, operate the switch to control the flow.
Generally, in another aspect, the invention features a system for bonding two materials together at a junction between the materials. The system includes an energy source, electrodes in contact with the materials, and a controller. The controller is configured to connect the energy source to the electrodes to transfer charge from the energy source to the junction, and disconnect the energy source from the electrodes after a predetermined amount of the charge has been transferred to the materials.
Generally, in another aspect, the invention features a system for bonding a number of first materials to a second material near different regions of the second material. The system includes an energy source and electrodes that are configured to establish charge flowpaths. The system also has controllers. Each different controller is associated with a different one of the flowpaths and is configured to cause charge to flow from the energy source through the associated flowpath until a predetermined amount of the charge flows through the associated flowpath.
Generally, in another aspect, the invention features a system for bonding glass spacer rods to a face plate of a flat panel display. The system includes an energy source, electrodes and controllers. The electrodes are configured to establish charge flowpaths. Each different flowpath is associated with a junction located between a different one of the glass spacer rods and the face plate. Each different controller is associated with a different one of the flowpaths and is configured to allow charge to flow from the energy source through the associated flowpath until a predetermined amount of the charge flows to the junction associated with the flowpath.
Generally, in another aspect, the invention features a method for anodically bonding two materials together. The method includes placing the two materials in contact with each other to form a junction between the materials. A current is applied through the materials to transfer charge to the junction. This current is monitored to determine the amount of the charge being transferred to the junction. The current is controlled based on the amount.
Generally, in another aspect, the invention features a method for bonding a number of first materials to a second material at different regions of the second material. The method includes placing each of the first materials in contact with the second material to form junctions between the first and second materials. Currents are applied through the first and second materials to transfer charge to the junctions. The amounts of charge transferred to each of the junctions are monitored, and based on the amounts, the currents are selectively controlled.
Generally, in another aspect, the invention features a method for anodically bonding slices of glass spacer rods to a face plate of a flat panel display. The face plate has a conductive layer for causing the emission of electrons from a base plate. The method includes placing the slices of glass spacer rods in contact with the face plate to create junctions between the slices of glass spacer rods and the face plate. An electrode is place in contact with each group of glass spacer rods to form a charge flowpath between each electrode and the conductive layer. A potential is applied between the electrodes and the conductive layer to cause charge to flow through the charge flowpaths. For each charge flowpath, an amount of charge flowing through the charge flowpath is monitored. The flow of charge through the flowpaths is selectively controlled based on the monitored amounts.
Generally, the invention features a method for bonding glass spacer rods to a face plate of a flat panel display. The method includes connecting electrodes to the face plate and glass spacer rods to establish charge flowpaths. Each different flowpath is associated with a junction located between a different one of the glass spacer rods and the face plate. An energy source is connected to the electrodes. For each flowpath, charge is allowed to flow from the energy source through the flowpath until a predetermined amount of the charge flows into the junction associated with the flowpath.
In implementations of the invention, the circuit may be configured to halt the flow of charge through the flowpath when the amount exceeds a predetermined threshold. The circuit may also be configured to operate the switch to halt the flow when the rate exceeds a predetermined level, and the circuit may also be configured to operate the switch to allow the flow to resume after a predetermined duration expires after the circuit halts the flow.
The circuit may include a timer that is configured to measure the predetermined duration. The circuit may include an integrator that is configured to determine the amount of charge supplied to the materials based on the integration of the rate over time. The circuit may include a comparator that is connected to the integrator and is configured to indicate when the amount of charge exceeds the predetermined threshold.
The materials may include an oxidizable material, such as an oxidizable material that is located on a face plate of a flat panel display. The materials may also include glass spacer rods of a flat panel display.
Other advantages and features will become apparent from the following description and from the claims.